Without adding fat, brown the lamb chunks in a heavy Dutch oven. Heat the oil in a separate pan and saute the onion in it until it is brown, then transfer to the Dutch oven. Sprinkle the turmeric, pepper and salt over the meat and stir for 1 minute. Sprinkle in the flour and stir until it is absorbed. Add the walnuts, pomegranate juice and enough water to almost cover the meat. Reduce the heat to a simmer, cover the pot and cook until the meat is tender, 1 to 1 1/2 hours. Add the cardamom. Squeeze in the lemon juice to taste and adjust the salt. Keep warm until ready to serve.